Oxidation of Au4Al in gold ballbonds

Abstract Two different 4N (99.99% purity) gold wires were ballbonded on 1 μm thick Al–1 wt.% Si–0.5 wt.% Cu bondpad metallisation and subjected to high temperature storage (HTS) at 175 °C in air. Each wire type showed ball lift failures, Type A after 500 h and Type B after 1500 h, which in both cases was a result of Au 4 Al oxidation. With wire Type A the dominant compound underneath the ball was Au 8 Al 3 . A thin layer of Au 4 Al (≈ 1 μm thick) was observed between the Au 8 Al 3 and the gold ball. Ball lift failures occurred in the Au 4 Al layer, which appeared to disintegrate due to oxidation and the resulting by products of oxidation were deposited on the underlying and unoxidised Au 8 Al 3 . With wire Type B, a double layer Au 4 Al was dominant after long term ageing and Au 8 Al 3 was confined to the ball periphery. Consequently, because of the much greater volume of Au 4 Al, compound oxidation resulted in the formation of a large amount of a completely new microstructure consisting of gold precipitates embedded in a dark oxide matrix. The Au 8 Al 3 compound remained unoxidised. It is speculated that internal stress and contamination may accelerate the oxidation reaction.